Power electronic circuits often comprise semiconductor switches that are used to supply a load with a power. In some instances high power losses may be dissipated in the semiconductor switches if a high current flows through a semiconductor switch and at the same time a high voltage is present across said component. Such power losses arise in particular during a switchover process.
In order to avoid damage to and/or an adverse effect on the lifetime of a semiconductor switch as a result of dissipated power loss, a switching frequency with that the semiconductor switch is switched over may be limited. However, such a limitation is at odds with the demand for the highest possible switching frequencies enabling control of a load to be improved.
A further possibility for reducing switching losses consists in a reduction of the switching time. However, under certain circumstances a higher switching time has a negative effect on the EMC behavior (electromagnetic compatibility EMC) of the circuit, and so higher switching losses are therefore often accepted in practice.
Furthermore, load relief networks can be used to ensure that the product of voltage and current is as low as possible during a switching process. By way of example, it is known to connect a capacitor in parallel with a transistor, that capacitor accepts a load current during switch-off, such that the transistor becomes free of current and may be switched off in a manner free of power loss.
U.S. Pat. No. 5,341,004 A proposes, in order to reduce the switching losses, interconnecting in parallel with a first semiconductor switch, an IGBT (insulated-gate bipolar transistor, IGBT), a second semiconductor switch, likewise an IGBT. The second IGBT has a higher saturation voltage and a shorter fall time than the first IGBT and thus accepts current when the first IGBT is turned off, such that the current no longer flows via the first IGBT and thus establishes a power loss.
DE 3542751 A1 proposes, in order to reduce the power loss in snubber networks for semiconductor switches, connecting a saturable tapped inductor in series with the semiconductor switch and providing an ohmic resistor in parallel therewith. What is achieved thereby is that the step current flow in the forward direction that flows when the semiconductor switch is switched on is lower and accordingly causes lower losses.
By the solutions mentioned, although it is possible to reduce a power loss directly in the semiconductor switch, a corresponding power loss may generally arise at other parts of the half bridge if those energy stores that are provided for reducing the power loss are drained. As a result, the effectiveness of the measures for reducing the power loss in the semiconductor switch may also be limited, e.g. if a corresponding power cannot be dissipated rapidly enough in a controlled manner.
DE693 12 585 T2 discloses a half bridge of the generic type comprising series-connected semiconductor switches and a load relief device in order to collect energy during a switching process in the semiconductor switches.